In recent years, methods of hydrolysis of a cellulose-containing biomass using enzymes, which methods use less energy and cause less environmental load but produce sugar at high yields have been extensively studied. However, methods using enzymes have a drawback in that the cost of the enzymes is high.
As methods of solving such a technical problem, methods in which an enzyme used for the hydrolysis is recovered and reused have been proposed. Among these methods, methods in which the enzyme adsorbed to the hydrolysis residue is recovered by washing with an aqueous alkaline solution are less costly and suitable for the purpose of reducing the cost of the enzyme. Thus, many studies have been carried out on such methods.
More specifically, a method in which the hydrolysis residue after enzymatic saccharification is washed with an aqueous sodium hydroxide solution at a pH of about 8 to recover the enzyme component from crystalline cellulose (D. E. Otter et al., “Elution of Trichoderma reesei Cellulose from Cellulose by pH Adjustment with Sodium Hydroxide,” Biotechnology Letters (1984), Vol. 6, No. 6, 369-374), and a method in which the enzyme adsorbed to crystalline cellulose or dilute sulfuric acid treated corn stover is recovered by washing with an aqueous alkaline solution at a pH of 10 to 13 (Z. Zhu et al., “Direct quantitative determination of adsorbed cellulose on lignocellulosic biomass with its application to study cellulose desorption for potential recycling,” Analyst (2009), Vol. 134, 2267-2272) are known.
As a result of studying conventional methods of recovering the enzyme, we discovered that the type of the recovered enzyme is biased, and that reusability of the recovered enzyme is not sufficient. In view of this, we found that it could be helpful to increase reusability of filamentous fungus-derived cellulase recovered from a hydrolysis residue of a cellulose-containing biomass obtained using the filamentous fungus-derived cellulase, to thereby reduce the amount of the filamentous fungus-derived cellulase used in the method of producing a sugar liquid.